A series of gnotobiotic experiments is being conducted to determin the relative cariogenicity of different serotypes of S. mutans and certain S. mutans isolated of interest. These monoinfected rat experiments follow a stringent standardized test regimen. Experiments are conducted sequentially as only one or two isolators are avilable at a time and breeding facitlities have limited the availability of 18-20 day old Osborne-Mendel rats. Replacement of the previous breeding diet has resulted in a significant increase in liter size. Results to date suggest that significant differences to exist in the cariogenicity of different strains. Careful analysis of caries scores has prompted the shortening of the experimental periods from eight to six weeks. Improved methods to insure a uniform innoculum from archieved strains of S. mutans have been adopted. Additional experiments are currently being conducted to determine the parameters of these differences in cariogenicity and the reproducibility of the standardized test conditions.